1. Field of the Invention
This invention relates to cardiovascular implants fabricated of zirconium or zirconium alloys that are coated with a thin layer of zirconium nitride or black or blue-black zirconium oxide to provide resistance to wear and enhanced biocompatibility. More specifically, the invention is of synthetic heart valves and artificial hearts fabricated of zirconium or zirconium alloy coated with a thin layer of zirconium nitride or black or blue-black zirconium oxide to reduce wear and enhance blood biocompatibility.
2. Description of the Related Art
Cardiovascular implants have unique blood biocompatibility requirements to insure that the device is not rejected (as in the case of tissue materials for heart valves and grafts for heart transplants) or that adverse thrombogenic (clotting) or hemodynamic (blood flow) responses are avoided. For mechanical devices, properties such as the surface finish, flow characteristics, surface structure, charge, wear and mechanical integrity all play a role in the ultimate success of the device. When these implants are fabricated from natural tissue, known as bioprostheses, they can be affected by gradual calcification and the eventual stiffening and tearing of the implant. Surface charge has been shown to play a key role in the propensity of these bioprostheses to form calcium phosphate deposits
Non-bioprosthetic devices (mechanical) are fabricated from materials such as pyrolytic carbon-coated graphite, pyrolytic carbon-coated titanium, stainless steel, titanium alloys, cobalt-chrome alloys, cobalt-nickel alloys, alumina coated with polypropylene and poly-4-fluoroethylene. Typical materials used for balls and disks for heart valves include nylon, silicone, hollow titanium, TEFLON.TM., polyacetal, graphite, and pyrolytic carbon. Artificial hearts are fabricated from various combinations of titanium, carbon fiber reinforced composites, polyurethanes, BIDLON.TM. (DuPont), Hemothane.TM. (Sarns/3M), DACRON.TM., polysulfone, and other thermoplastics. One of the most significant problems encountered in both heart valves and artificial hearts is the formation of blood clots. Protein coatings are sometimes employed to reduce the risk of blood clot formation.
It has been found that stagnant flow regions also contribute to the formation of blood clots. These stagnant regions can be minimized by optimizing surface smoothness and minimizing abrupt changes in the size of the cross section through which the blood flows or minimizing either flow interference aspects. While materials selection for synthetic heart valves and cardiac implants generally is therefore dictated by a requirement for blood compatibility to avoid the formation of blood clots, cardiovascular implants must also be designed to optimize blood flow and wear resistance.
Even beyond the limitations on materials imposed by the requirements of blood biocompatibility and limitations to designs imposed by the need to optimize blood flow, there is a need for durable designs since it is highly desirable to avoid the risk of a second surgical procedure to implant cardiovascular devices. Further, a catastrophic failure of the device will almost certainly result in the death of the patient.
The most popular current heart valve designs include the St. Jude medical tilting disk double cusp (bi-leaf) valve. This valve includes a circular ring-like pyrolytic carbon valve housing or frame and a flow control element which includes pyrolytic carbon half-disks or leaves that pivot inside the housing to open and close the valve. The two leaves have a low profile and open to 85.degree. from the horizontal axis.
Another popular heart valve is the Medtronic-Hall Valve wherein the flow control element is a single tilting disk made of carbon coated with pyrolytic carbon which pivots over a central strut inside a solid titanium ring-like housing. A third, less popular design, is the Omniscience valve which has a single pyrolytic disk as a flow control element inside a titanium housing. Finally, the Starr-Edwards ball and cage valves have a silastic ball riding inside a cobalt-chrome alloy cage. The cage is affixed to one side of a ring-like body for attachment to the heart tissue.
The St. Jude and Medtronic-Hall valves appear to be best suited to maximize hemodynamic performance from a design standpoint. However from the point of view of durability, these heart valves could fail from disk fracture related to uneven pyrolytic carbon coating, fracture of the ball cage, disk impingement, strut wear, disk wear, hinge failure, and weld failure.
A more recent heart valve, the Baruah Bileaflet is similar to the St. Jude design but opens to 80.degree. and is made of pure zirconium. The valve has worked well over its approximately two year history with roughly 200 implants to date in India. This performance can be partly attributed to the lower elastic modulus of zirconium (about 90 GPa) and the resultant lower contact stress severity factory (Cc of about 0.28.times.10.sup.-7 m) when the disk contacts the frame. In contrast, pyrolytic constructions produce contact stress severity factors of about 0.54.times.10.sup.-7 m.
Although zirconium has worked well to date and can reduce contact stress severity, zirconium metal is relatively soft and sensitive to fretting wear. This is partly due to the hard, loosely attached, naturally-present passive oxide surface films (several nanometers in thickness) which can initiate microabrasion and wear of the softer underlying metal. However, this naturally present zirconium oxide passive film is thrombogenically compatible with blood and the design is acceptable from a hemodynamic standpoint. Therefore, while the zirconium bileaflet valve appears to meet at least two of the major requirements for cardiac valve implants, namely blood compatibility and design for minimum stagnant flow regions, the use of soft zirconium metal leads to a relatively high rate of fretting wear and leads to the expectation that the valve may be less durable than one produced from materials less susceptible to fretting wear.
There exists a need for a metallic cardiac valve implant that is biocompatible, compatible with blood in that it does not induce blood clotting and does not form a calcified scale, that is designed to minimize stagnant flow areas where blood clotting can be initiated, that has a low elastic modulus for lower contact stress severity factors to ensure resistance to wear from impact and that has a surface that is also resistant to microabrasion thereby enhancing durability.